Solid state image sensors comprise a grid of laterally spaced sensor elements, each of which is called a "pixel". Each pixel in the sensor includes a photosensor such as a photodiode or a photocapacitor. In operation, light impinging upon an active silicon region in the pixel results in the generation of a charge. This charge, generated in each pixel, can be read and used to construct a digital image. Semiconductor arrays including sensor elements of this type are usually prepared by sequential patterning of conductors, semiconductors and insulators.
Silicon photosensors employed in these devices are sensitive to a wide bandwidth of radiation including the visible spectrum. However, such sensors are unable to separate the light into the several colored components used to reconstruct an image for printing, electrostatic imaging, thermal printing, silver halide imaging or video display. These technologies generally require several different color separated inputs, typically red, green and blue or cyan, magenta and yellow.
Heretofore, color separation has been achieved by constructing an array of dyed patterns which are to be placed upon the top of a sensor array. This array can be constructed externally and positioned atop the device or, alternatively, may be prepared directly upon a completed device wafer. This latter method for fabricating an integral color filter array (CFA) is a preferred approach for preparing high resolution devices because of the ease of accurately aligning the CFA to the pixels in the sensor array. CFA's employed for this purpose comprise layers of polymers which have been patterned and contain a dye. A typical pixel may be covered by a single polymer layer containing one or several dyes or by several layers of polymer, each of which contains one or more dyes. These dyed layers serve to remove light at certain wavelengths while permitting light at other wavelengths to reach the photosensor. Careful selection of dyes and an array pattern permits light at different wavelength to be sampled separately and the construction of a color separated image.
A preferred technique for the construction of an integral CFA involves coating and patterning a photosensitive polymer imaging system and, subsequently, dyeing the image. The coating, exposure and development of the photopolymer system are analogous to the operations performed using a photoresist for microlithography in the fabrication of devices. However, these images will be dyed and will remain in place as part of the finished device rather than being removed from the device. The image in this process may be dyed by placing the wafer in contact with a solution of dye or dyes which can migrate into the polymer image and be held or mordanted there. The rate of dye uptake and the amount of dye taken into the polymer are functions of the composition of the polymer and the dye, the process for making the image, and the solvents used to make the dye solution.
Still another method for the fabrication of a CFA involves using a photoresist having a dye therein prior to patterning. This obviates the necessity for dyeing solutions and processes. However, the viability of this technique is limited by the economics of using several different dyed resists and the distinct possibility of dye interference with resist exposure.
In both of the foregoing methods, the final intensity of the image is strongly dependent upon the thickness of the coating being dyed. Accordingly, at the saturation point, a thick coating absorbs more light whereas a thin coating will absorb less. Thus, the patterning process is repeated several times using different dye solutions or dyed resists.
Studies have revealed that a uniform pixel response is attainable only if the thickness of the dyed CFA coating is uniform throughout the device. This, of course, would not create any problems on a flat homogeneous surface. However, a completed device is not flat. The processing steps required to make the image sensor typically involve the patternwise definition of materials, either involving etching or deposition. Due to the patternwise definition of materials during the fabrication sequence, a finished wafer containing a plurality of integrated circuits or image sensors is no longer microscopically smooth and evidences topographical features of varying height or depth and of varying lateral dimension. These topographic features frequently evidence variations in vertical dimensions ranging from about 0.2 to 10 micrometers.
In the operation of the process for the preparation of such CFA coatings, a film is typically deposited by spin coating a solution or suspension on a wafer having topographic features of the type noted above. Under these circumstances, it is common for the film so deposited to exhibit thickness variations extending radially outward from the major topographic features which may be inherent in the device geometries or arise from test or alignment structures. These radially directed thickness variations are commonly referred to as radial streaks which are nothing more than a physical response to the effort to compel a thin layer of fluid to flow over a tall obstacle. When the wafer is subjected to spinning, the fluid is thrown outwardly from the center of the wafer and when it impinges on a topographic barrier which rises from the surface, the flow is disrupted and the coating solution preferentially goes around the feature rather than over it. This results in the formation of a shadow of thin coating behind the feature with thick edge beads on either side of the shadow. This combination of features is a streak.
A plurality of image sensor chips (called die) are simultaneously fabricated on a wafer. The die are positioned on the wafer so as to maximize the number of high quality chips produced. The die are separated by a grid commonly termed a street. The finished die are then separated into discrete chips by sawing or snipping the wafer along the streets. Since the street is considerably lower in height than the active area of the die, coating solutions tend to flow and pool in these areas rather than uniformly coating the entire wafer surface. Examination of a cross-section of the wafer reveals that there is an abrupt change in going from a street to the device surface. Accordingly, the active light collecting area of a pixel may range from 0.2 to 1.5 micrometers below the top of the device, while the street may range from 2 to 7 micrometers below the top of the device.
Studies have revealed that during the polymer coating process, streaks can be caused by any topographical feature. Prominent streaks have been observed at the inner corners of the die where the coating solutions tend to follow the street rather than climbing over the corner of the die. Streaks tend to point away from the center of the wafer and comprise a center film with thicker areas surrounding it. In light of the fact that the CFA remains on the finished device, any defects, such as streaks in the CFA, tend to adversely affect the quality of the sensor performance. Thus, photosensors under thick areas evidence less response to light and appear dark on pictorial output while thin areas allow light of other wavelengths to register and, therefore, appear too bright. The image of the streak then appears in the output image.
In order for a device chip to function in an electronic system, it must receive input and send output byway of connections to bonding pads. Accordingly, it is a constraint of the device and CFA fabrication that the metal bonding pads must be accessible for wire bonding, etc. when fabrication is completed. These open bonding pads contribute only slightly to the streaking phenomenon, but the processes for keeping the bonding pads open greatly increase the difficulties of both planarization and CFA fabrication.
Still another problem encountered with respect to thin CFA films arises when sharp topographic features are present during CFA fabrication. A coating which is forced to go over a sharp edge is both thinner than normal and under considerable stress. Subsequent swelling, during development or dyeing and shrinkage during baking, may result in cracking of the coating which exposes active regions near the crack to unfiltered light and creates particulate defects.
And lastly, another problem encountered in the construction of color filter arrays (CFA's) on a topographic wafer is that the thickness of the dyed coating varies across the width and length of each pixel. Dependent upon the topography of the finished sensor device, the coating at the center of the photoactive silicon area maybe either thicker or thinner than at the edge of the active area.